This invention relates generally to a laser ablatable material and its use and, more particularly, to an epoxy base laser ablatable material and its use to form a dielectric substrate having micro vias therein
One typical technique for forming a dielectric layer of material with vias therein is to utilize an epoxy base, photoimageable material coated onto a substrate, such as another dielectric material or a conductive material, and form the vias or other openings therein by photolithographic techniques. Particularly suitable materials are described in U.S. Pat. No. 5,026,624 and 5,300,402, and U.S. patent application Ser. No. 09/212,204, filed Dec. 15, 1998, all commonly assigned, and all of which are incorporated herein by reference. As described in these patents and patent application, the photoimageable material is coated onto a substrate and U.V. light of the appropriate wavelength is used to expose the desired pattern on the photoimageable material, after which the material is developed to form the desired vias and other openings in the material.
For many applications, this is a very successful technique. However, in specific applications, certain difficulties may be encountered in using photolithographic techniques. For example, in certain applications, copper foil is laminated to a dry film of material made according to the teachings of either U.S. Pat. No. 5,665,650 or 5,670,750 and results in very narrow process windows and manufacturing limitations. In particular, to achieve optimum adhesion of the foil to the dielectric material, a low exposure dose is required. However, too low an exposure will induce defects upon stressing, and too high an exposure dose will result in degradation of the bond between the foil and the dielectric material. Thus, in some cases, an optimum dosage to achieve good peel strengths is not adequate for small micro via definition. Moreover, when exposure through artwork is required, defects, such as dust particles, scratches etc., will print through, leaving a defect in the dielectric layer. With laser ablation, photo induced defects will not appear on the dielectric layer.
According to the present invention, an epoxy based resin which exhibits good laser ablation and good adherence to a substrate, such as copper, is provided by adding to the resin a dye or dyes having substantial energy absorption at the emission wave lengths of lasers used to laser ablate the resin. In one embodiment, the resin is coated onto a substrate and cured, or laminated onto a substrate. Then the cured film is laser ablated to form the desired pattern of openings. The film can also be either laminated or coated and cured onto a copper foil, and the dielectric with the copper foil thereon laminated to a substrate, This allows for the use of optimum techniques to be used to form micro vias.